basically, "foss" stands for "free and open source software". it's really awesome because it's free, it's open source which means if a project dies another developer can fork the code and start a new project based on the old project, and it doesn't have any restrictive copyright, along with a LOT of other cool reasons. i suck at explaining things though, so i recommend you read the wikipedia article about the subject. anyway, i always try my best to stick to using free and open source software, so here's my list of the software i use that's classified as that! i encourage you to try some of these as well :)

vlc media player (for my music)
mpv player (for videos)
odyssey (phone music player)
youtube-dlg (downloading videos)
newpipe (yt on my phone)

waterfox (my main browser)
firefox klar (phone browser)
firefox (previously used)

libreoffice (replacement for microsoft office)
notepad++ (for coding)
sumatrapdf (reading/opening pdfs)

rainmeter (displaying desktop widgets)
keepassxc (password manager)
qbittorrent (for torrents)
audacity (for simple/quick audio edits)
flym (rss reader on my phone)
simple clock (phone clock/alarm)
simple calculator (phone calculator)
simple notes (phone notes)
weather (phone weather)
waveup (turning on my phone thing)
f-droid (foss app store for android)